The formal introduction of the Datamath 2500 calculator was September 1972. However a very limited number of calculators were sold in some department stores in Dallas, TX a couple of months before. It is estimated that about 500 units were sold in advance. These units use the white case of the prototype series, have a different color scheme and have a different back label than the first production series.
Mpre details about this at Datamath Museum site here.
I am the lucky owner of one of these units with the serial number 00000599. Below are some images.
For comparison please see below the first version of the calculator released in September 1972:
